Miniaturization and integration of electronic devices have caused the flood of electromagnetic waves even at work and home together with rapid supply of PC, mobile phones, and digital devices, and thus electronic industries have been developed but the threat of electromagnetic interference has been more growing.
The electromagnetic interference variously appears ranging from false operations of computers to burnout accidents of factories, and further, research results about adverse effects of the electromagnetic interference on human bodies are represented one after another, so that concerns and interests for health have increased. In this situation, restrictions on the electromagnetic interference have been strengthened and measures therefor have been prepared.
Since an EMI shielding effect is directly proportional to electroconductivity, metals are generally used currently as most EMI shielding materials.
Metal materials reflect electromagnetic waves, but insulating materials such as plastics and the like allow electromagnetic waves to pass therethrough. EMI shielding has been widely known. When electromagnetic waves reach an electrical conductor, some of them are absorbed to and passed through the metal but most of them reflect from a surface of the metal, without entering the metal. The reason is that, when the electromagnetic waves reach the conductor, eddy current is generated in the conductor due to electromagnetic induction, which reflects the electromagnetic waves.
Many attempts have been made to impart electroconductivity to a resin composition, which is then used to allow vehicles and various kinds of electric apparatuses, electronic assemblies, or cables to exhibit EMI shielding performance or the like.
This electroconductive resin composition is prepared by generally mixing a conductive additive, such as, carbon black, carbon fiber, metal powder, metal coated inorganic powder, or metal fiber, to a polymer. However, it is difficult to secure a sufficiently desired degree of electroconductivity of the electroconductive resin composition so long as a considerable amount of the conductive additive is not added.
In addition, in the case of a polymer composite using carbon materials such as carbon black, carbon fiber, or the like, a large amount of inorganic material is input, resulting in higher hardness, surface roughness, and deterioration in physical properties, of the resin, and thus it is difficult to realize high conductivity required.
Meanwhile, attempts to impart excellent electroconductivity to the electroconductive polymer by using a carbon nanotube as a conductive additive was made.
However, in the case where an electroconductive composite is obtained by mixing the carbon nanotube to the polymer and performing extrusion-ejection, the carbon nanotubes are agglomerated or aligned due to shear stress generated during the extrusion-ejection process and the carbon nanotubes are defectively dispersed in the electroconductive polymer, and thus it is difficult to obtain a sufficiently desired degree of electroconductivity.
As described above, the related art has problems of deterioration in dispersibility, excessive amounts of additives used, and not meeting the desired degrees of physical properties.
Korean Patent Laid-Open Publication No. 2011-0000296 discloses “a fabrication method of polymer/carbon nanotube composite and a polymer/carbon nanotube composite manufactured by the method, the polymer/carbon nanotube composite having good electromagnetic interference shielding efficiency, the method comprising: a) dissolving a first polymer resin and a carbon nanotube (weight ratio of 100:5 to 100:30) in a solvent and performing ultrasonication, to prepare a preliminary composite; and b) mixing the preliminary composite and a second polymer resin (weight ratio of 1:1 to 1:15) in an extruder”.
In addition, Korean Patent Laid-Open Publication No. 2011-0104456 discloses “an electroconductive resin composition for being used as a molding material of information technology devices requiring an electric wave transmission and reception function, the electroconductive resin composition comprising: a) 30˜60 wt % of a polymer resin; b) 3˜15 wt % of carbon black and 2˜10 wt % of a carbon nanotube, as a conductive material; and c) 30˜55 wt % of a carbon fiber for electrically connecting between the conductive carbon black and the carbon nanotube”.
However, according to the technologies disclosed in these patent documents, it is still difficult to disperse the carbon nanotube in the resin in order to maximally exhibit performance of the carbon nanotube, and a large amount of carbon nanotube needs to be input to exhibit EMI shielding effect, and thus high-priced raw materials are more used as compared with the existing carbon materials such as carbon black or carbon fiber, resulting in low economic feasibility.